Objectives: Vaccination remains one of the crucial countermeasure strategies against the intentional use of Bacillus anthracis. Assessment of the cell-mediated immune response to the licensed UK vaccine, Anthrax Vaccine Precipitated (AVP), particularly in persons receiving a booster vaccination, is needed for determining the optimal utilisation of this vaccine. We are conducting a prospective, observational study using an Enzyme-Linked Immunospot (ELISPOT) assay to assess the cell-mediated immune response before and after booster vaccination.

Methods: Laboratory workers scheduled to receive their yearly AVP booster vaccination had blood samples obtained before vaccination, then at 1, 2, 4 and 8 weeks after vaccination. Peripheral blood mononuclear cells were extracted from whole blood samples, and BDTM ELISPOT kits (Becton Dickinson) for IFN-g, IL-4 and IL-5 were used to assess cellular responses to antigen-specific stimulation (5×105 cells per well). Antigens included protective antigen (PA), lethal factor (LF), and edema factor (EF), with concanavalin-A as a positive control, and tetanus toxoid as an additional control. A positive response was defined as a stimulation index greater than three times the number of positive cells in the medium only (negative control) wells.

Results: Prior to booster vaccination, the following number of subjects had positive responses with IFN-g: 2/5 for PA stimulation, 3/4 for LF, and 2/5 for EF. Two subjects had more than a three-fold increase in their IFN-g spot counts with PA stimulation by day 56 after vaccination (compared to pre-vaccination spot counts), while three other subjects had no change. These subjects had similar trends in IFN-g spot counts with LF and EF stimulation. Four subjects had low baseline responses to IL-5 and IL-4 across all of the anthrax antigens, with some increase in IL-5 noted after vaccination but minimal changes observed in IL-4 counts.

Conclusions: In this ongoing study, cell-mediated responses to anthrax antigens were measurable in approximately ½ of subjects one year after their last anthrax vaccination. Some subjects were observed to have a substantial increase in their cell-mediated response to these antigens, while other subjects had no response. Further testing is needed to assess the factors that influence these immune responses.